1. Field of the Invention
The present invention relates to a solid polymer electrolyte membrane type fuel cell and to a fuel cell stack constituted by stacking a plurality of the fuel cell units, and more specifically, relates to a technique for absorbing expansion and contraction of the fuel cell stack in the stacking direction of separators.
2. Description of the Related Art
Fuel cells include a solid polymer electrolyte membrane type fuel cell constituted by providing a pair of electrodes on opposite sides of the solid polymer electrolyte membrane and sandwiching the outside thereof by a pair of separators.
In this fuel cell, a passage for a fuel gas (for example, hydrogen) is provided on the entire surface of a separator provided facing one electrode, a passage for an oxidant gas (for example, air including oxygen) is provided on the entire surface of a separator provided facing the other electrode, and a passage for a cooling medium is provided on either one of the surfaces of separators opposite to a surface facing the electrode.
When the fuel gas is supplied to the reaction surface of one electrode, hydrogen is ionized and moves to the other electrode via the solid polymer electrolyte membrane. Electrons generated during the reaction process are taken out to an external circuit, and are used as direct-current electrical energy.
Since the oxidant gas is supplied to the other electrode, the hydrogen ions, the electrons and the oxygen react with each other to thereby generate water.
The surface on the opposite side of the electrode reaction plane of the separator is cooled by the cooling medium flowing between the separators.
Since these reactant gases and the cooling medium should flow in respectively independent passages, a sealing technique, which separates each passage, is important.
The portions to be sealed include, for example, the peripheries of communication holes formed penetrating through the separator so as to distribute and supply the reactant gas and the cooling medium to each fuel cell unit in the fuel cell stack, the outer peripheries of membrane electrode assembly formed of the solid polymer electrolyte membrane and a pair of electrodes arranged on opposite sides thereof, the outer peripheries of a coolant passage plane of the separator, and the outer peripheries of front and back faces of the separator. As the sealing material, an elastic and adequately resilient material, for example, an organic rubber, is adopted.
Conventionally, a fuel cell having a membrane electrode assembly by sandwiching a solid polymer electrolyte membrane by a pair of electrodes and sandwiching the outside thereof by a pair of separators, comprises a membrane electrode assembly (as shown in FIG. 17) constituted by sandwiching a solid polymer electrolyte membrane having a larger outer size between a pair of gas diffusion layers each having the same size, and the outer size thereof is smaller than that of the solid polymer electrolyte membrane, as disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 8-148169. In this type of fuel cell 40, the passage for the fuel gas 41 and the passage for the oxidant gas 42 are sealed by sandwiching with a pair of O-ring the portion of the solid polymer electrolyte membrane 45, which is protruded from the outer peripheries of the anode electrode 43 and the cathode electrode 44.
However, in such a sealing structure, a problem arises in that sealing of passages may fail if a pair of O-rings each disposed on both side of the solid polymer electrolyte member are insufficiently aligned.
For example, as shown in FIG. 18, if two O-rings on both surface of the solid polymer electrolyte membrane are disposed out of positions, the pressure of both O-rings press the solid polymer membrane and the solid polymer electrolyte membrane 45 is be deformed such that the surface pressure of the O-rings becomes insufficient to provide a hermetic seal. In addition, an unfavorable phenomenon will be caused by deformation of the solid polymer electrolyte membrane in that the solid polymer electrolyte membrane will be peeled off from the anode electrode 43 and the cathode electrode 44.
In order to avoid such unfavorable phenomena, the grooves to align O-rings must be formed in a very precise manner, which results in increasing the manufacturing cost.
Since the fuel cell 40 is used as a fuel cell stack after stacking a plurality of fuel cell units, the thickness of the fuel cell unit is desired to be as thin as possible.
An object of the present invention is to solve the aforementioned problems and to provide fuel cell units and a fuel cell stack formed by stacking a plurality of fuel cell units, wherein those fuel cell units and a fuel cell stack have an improved sealability between the membrane electrode assembly and separators, a reduced cost, and an improved thickness in the direction of stacking.
In order to solve the above problems, the first aspect of the present invention provides a fuel cell which comprises a pair of separators (for example, a first separator 3 and a second separator 4 in the embodiment) sandwiching outsides of a membrane electrode assembly composed of a pair of electrodes provided on both sides of a solid polymer electrolyte membrane, an outer seal member sandwiched by a pair of separators at a position surrounding an outer periphery of the membrane electrode assembly, an inner seal member sandwiched by one (for example, the second separator 4 in the embodiment) of the pair of separators and an outer periphery of the electrolyte membrane, and a backing member (for example, an anode electrode in the embodiment) opposing to the inner seal member interposing the electrolyte membrane, wherein a step is formed at contact surfaces (a first plane portion 22 and a second plane portion 23 in the embodiment) of the inner seal member and the outer seal member on one of the pair of separators.
In the fuel cell according to the first aspect of the present invention, since the outer seal member surrounding the periphery of the membrane electrode assembly tightly seals a space between the first and second separators, and the inner seal member which is arranged at the outer periphery of the electrolyte membrane constituting the membrane electrode assembly at the inner side of the outer seal member tightly seals the space between the separator and the electrolyte membrane, the space between separators are separated on both side of the electrolyte membrane into two separated regions, one of which is located at the anode side and the other one is located at the cathode side.
Since the inner seal member and the second separator sandwiches the electrolyte membrane together with the backing member, the thin electrolyte membrane is reinforced by the backing member so that it can be protected from being deformed by a pressure through the inner seal member. In addition, since the electrolyte membrane is not deformed, the inner seal member is able to maintain sufficient surface pressure to attain the sufficient sealing ability.
In the above case, in contrast to the outer seal member tightly which seals the space between two separators, the inner seal member performs tight sealing between two separators including the electrolyte membrane and the backing member in between, so that a difference in thickness is generated between the inner seal member and the outer seal member. For example, even when the thickness of the inner seal member is set to a minimum thickness including a deformable portion for sealing, the thickness of the outer seal member which must seal a wider space than that for the inner seal member becomes excessively thick.
In both separators in contact with the outer seal member and the inner seal member, if the heights of the contact planes with both seal members are changed, that is, if a step is formed between respective contact planes, it becomes possible to reduce the thickness of the outer seal member, which results in reducing the material consumption of the seal member and reducing the product cost. In addition, since it is possible to reduce the thickness of the outer seal member while maintaining the necessary thickness for the inner seal member, it is possible to reduce the thickness of the fuel cell unit.
In a fuel cell according to the second aspect of the present invention, the anode electrode or the cathode electrode is used as the backing member of the electrolyte membrane.
According to the second aspect of the present invention, the fuel cell uses a backing member of the electrolyte membrane the anode electrode or the cathode electrode, without using a separate element.
Application of the anode electrode or the cathode electrode to the electrolyte membrane without using a separate element as the backing member makes it possible to reduce the number of parts in the fuel cell and to reduce the product cost.
According to the third aspect of the present invention, the present invention proposes to use the second separator as a backing member for reinforcing the electrolyte membrane.
Application of the second separator to the electrolyte membrane as the backing member without using a separate element makes it possible to reduce the number of parts in the fuel cell and to reduce the product cost.
The fourth to sixth aspects of the present invention propose a fuel cell stack, formed by stacking a plurality of fuel cells according to one among the first to the third aspects.
Since the thickness of the individual fuel cell is reduced as described above, the thickness of the fuel cell stack can be reduced by an amount corresponding to the reduced thickness for a fuel cell times the number of stacked fuel cells for forming the fuel cell stack.